The present invention relates to a method of manufacturing silicon wafers to be used in manufacturing semiconductor devices, suitable for non-oxidative heat treatment such as for example hydrogen atmosphere high-temperature heat treatment (called hydrogen heat treatment) etc.
Void defects etc that are agglomerated by vacancies and detected as for example LSTD, FPD or COP are present in silicon wafers (CZ-silicon wafers) manufactured by the Czochralski method (hereinbelow referred to as the CZ method). Since such crystal defects have an adverse effect on the quality of the final product, heat treatment such as hydrogen atmosphere high-temperature heat treatment (also called hydrogen heat treatment or hydrogen annealing) is often performed in order to remove such crystal defects. In fact it is known (Examined Japanese Patent Publications 3-80338) that in CZ-silicon wafers subjected to hydrogen treatment, void defects agglomerated by vacancies detected as LSTDs etc are eliminated, with the result that excellent oxide film voltage-withstanding properties are displayed.
However, since there is the problem that the benefit of hydrogen heat treatment is restricted exclusively to the extreme surface vicinity of the wafer, and noting the fact that the defect-eliminating benefit by the hydrogen heat treatment increases as the defect size becomes smaller, the method has been proposed (Unexamined Japanese Patent Publication No. 10-208987) of extending the benefit of the hydrogen heat treatment to deeper regions by speeding up the rate of cooling in the temperature zone where defects are generated during crystal growth so as to reduce the size of the defects to a very small size and performing hydrogen heat treatment thereon.
In addition, in order to make it possible to cope with increase in crystal diameter by this method, a method has also been proposed (Unexamined Japanese Patent Publication No. 10-260666) of achieving good wafer quality by optimizing V/G (where V is the pulling speed and G is the temperature gradient in the axial direction of the crystal in the vicinity of the melting-point), which governs the concentration of point defects (voids), which are thought to be the source of defects.
Recently, as another approach relating to reducing the size of defects, it is reported (Unexamined Japanese Patent Publication No. 10-98047) that wafers suitable for annealing can be manufactured by reducing the defect size by addition of nitrogen during growth of CZ silicon single crystals.
However, when nitrogen is added during crystallization from the melt, the nitrogen concentration changes in the length direction of the crystal due to segregation, so the problem arises that this tends to cause a nonuniform distribution of defects as a result.
Furthermore, although it is true that, when nitrogen is added, the benefit of high-temperature annealing can be more easily manifested, due to decrease in defect size. However, since the defect density increases on the other hand, under inadequate high-temperature annealing conditions, there is contrariwise a risk of causing deterioration of wafer quality.
To summarize, it has still not yet been fully verified whether or not wafers in which defect size has been reduced by addition of nitrogen (nitrogen doping) as described above are indeed suitable for use as products, and it has in fact not yet been clarified whether nitrogen-doped wafers can indeed be used as wafers for semiconductor device manufacture or not.
The present invention has been made in view of the foregoing problems, and an object of the present invention is, in a method of manufacturing a silicon wafer by doping with nitrogen, to verify manufacturing conditions such that a silicon wafer having satisfactory properties for use as a semiconductor device can be manufactured, and to enable manufacture of a nitrogen-doped wafer having excellent characteristics as a wafer for manufacturing a semiconductor device after performing heat treatment.
As a result of detailed study of growth conditions in view of the aforementioned problems, the inventors of the present application discovered manufacturing conditions for silicon wafers that have fully satisfactory performances for use in advanced semiconductor devices even after heat treatment, and thereby perfected the present invention.
More specifically, when the present inventors evaluated wafer performance through the measurement of gate oxide integrity of wafers doped with nitrogen (nitrogen-doped wafers), they discovered that, with this process, while nitrogen-doped wafers tended to show excellent results in a TZDB (time zero dielectric breakdown) test compared with wafers that had not been subjected to nitrogen doping after heat treatment under a non-oxidizing atmosphere, if the wafers were doped with nitrogen to a high concentration, the results of the TDDB (time dependent dielectric breakdown) test showed abnormalities.
Also, the present inventors discovered that, whereas whether the TZDB test results are good or not and whether abnormalities appear in the TDDB test results or not depend on the nitrogen concentration of the nitrogen-doped wafer, at the same time, the TZDB test results depend on the type of gas with which non-oxidative heat treatment is performed (for example whether this is hydrogen gas or argon gas), yet the nitrogen concentration for which abnormalities appear in the TDDB test results does not depend on the type of gas which is used to perform the non-oxidative heat treatment, but, rather, can be inferred to be practically fixed.
With these discoveries that made a great contribution to perfecting the present invention, the present inventors present for the first time in the present application the fact that nitrogen-doped wafers containing nitrogen of concentration not more than 4xc3x971014 atoms/cm3, which is a concentration at which abnormalities appear in the TDDB test after heat treatment under non-oxidative atmosphere, are preferable as silicon wafers for non-oxidative heat treatment for semiconductor device manufacture. They therefore have taken this as the basic content of the claims of the present application.
Also, when nitrogen-doped wafers were evaluated in terms of the non-defective ratio with respect to gate oxide film voltage-withstand properties (GOI), the benefit of hydrogen heat treatment was fully displayed at the surface. In addition this benefit was independent of the amount of nitrogen doping. However, when an evaluation was conducted regarding a portion at a certain depth, the gate oxide film non-defective ratio was found to depend on nitrogen concentration and in fact there is an upper limit and a lower limit for the nitrogen doping amount. Thus it became clear that, when used as a product for a semiconductor device, the nitrogen concentration must be within a prescribed range. This discovery also contributed greatly to perfecting the present invention.
It should be noted that although the aforementioned Unexamined Japanese Patent Publication No. 10-98047 has the statement xe2x80x9ca nitrogen concentration of at least 1xc3x971014 atoms/cm3xe2x80x9d, no information in support of this is disclosed.
More specifically, according to the present invention, methods as indicated below and silicon wafers for non-oxidative heat treatment for semiconductor device manufacture are proposed.
A silicon wafer for non-oxidative heat treatment for semiconductor device manufacture wherein the nitrogen concentration is in the range from 5xc3x971013 atoms/cm3 to 1xc3x971015 atoms/cm3, preferably the range from 5xc3x971013 atoms/cm3 to 8xc3x971014 atoms/cm3, even more preferably 5xc3x971013 atoms/cm3 to 4xc3x971014 atoms/cm3 and further more preferably the range from 1xc3x971014 atoms/cm3 to 4xc3x971014 atoms/cm3.
Specifically, considering only the gate oxide film voltage-withstanding capability as found by TZDB, the range of nitrogen concentration for a good quality product corresponds to from 5xc3x971013 atoms/cm3to 1xc3x971015 atoms/cm3 (within a range of gate oxide film withstand-voltage non-defective ratio of at least 90% as found by TZDB), preferably within a range of nitrogen concentration of 1xc3x971014 atoms/cm3 to 8xc3x971014 atoms/cm3 (within a range of gate oxide film withstand-voltage non-defective ratio of at least 95% as found by TZDB); if, in addition, the fixed current value TDDB is taken into consideration, this must be 4xc3x971014 atoms/cm3 or less. The range for satisfactory products can therefore be divided into four, namely, a range of from 5xc3x971013 atoms/cm3to 1xc3x971015 atoms/cm3; a range of 5xc3x971013 atoms/cm3 to 8xc3x971014 atoms/cm3; a range of from 5xc3x971013 atoms/cm3 to 4xc3x971014 atoms/cm3; and a range of from 1xc3x971014 atoms/cm3 to 4xc3x971014 atoms/cm3. These are suitably selected in accordance with the product to be manufactured.
It should be noted that the types of silicon wafer for heat treatment may include silicon wafers for hydrogen heat treatment supplied for heat treatment under a hydrogen atmosphere, silicon wafers for argon heat treatment supplied for heat treatment under an argon atmosphere, or silicon wafers for mixed gas supplied for heat treatment under a mixed gas atmosphere of hydrogen and argon; however, all non-oxidative heat treatments for reducing crystal defects of the wafer surface layer (annealing treatments under conditions in the absence of oxygen) are included in the scope of the present invention.
In a method of manufacturing a silicon ingot by pulling a silicon single crystal by the Czochralski method, a method of manufacturing a silicon ingot by pulling a silicon single crystal under conditions such as to form a portion of nitrogen concentration from 5xc3x971013 atoms/cm3 to 4xc3x971014 atoms/cm3 by doping of nitrogen. In particular, in a method of manufacturing a silicon ingot by pulling a silicon single crystal by the Czochralski method, a method of manufacturing a silicon ingot for manufacturing silicon wafers for non-oxidative heat treatment by pulling a silicon single crystal under conditions such as to form a portion of nitrogen concentration from 1xc3x971014 atoms/cm3 to 4xc3x971014 atoms/cm3 by doping of nitrogen.
In one embodiment of the present invention, a silicon wafer for heat treatment for semiconductor device manufacture as described above is manufactured by the Czochralski method (CZ method). In this case, a silicon ingot was manufactured by the Czochralski method by pulling a silicon single crystal while doping with nitrogen so as to provide a nitrogen concentration in a portion or the whole thereof of 5xc3x971013 to 1xc3x971015 atoms/cm3. Then, a portion of nitrogen concentration in the range 5xc3x971013 to 1xc3x971015 atoms/cm3, preferably 1xc3x971014 atoms/cm3 to 8xc3x971014 atoms/cm3 was cut from this silicon ingot and used for a silicon wafer for non-oxidative heat treatment for semiconductor device manufacture and in particular for a silicon wafer for hydrogen heat treatment or a silicon wafer for argon annealing. If the CZ method is employed, a system in which a magnetic field is applied to the melt (MCZ method) may also be adopted.
As the method of nitrogen doping, all methods that are currently known, such as the method of admixing nitrogen with the argon gas that is passed through the furnace when growing the crystal or the method of introducing nitrogen atoms into the pulled single crystal by dissolving silicon nitride in the raw-material melt and all methods that may in future be discovered can be employed.
Also, as embodiments of the present invention, the following may be cited.
A silicon wafer for semiconductor device manufacture manufactured by performing hydrogen heat treatment or argon annealing on a silicon wafer (silicon wafer for non-oxidative heat treatment as referred to above) for non-oxidative heat treatment for semiconductor device manufacture according to the present invention.
A silicon wafer for semiconductor device manufacture wherein the amount of nitrogen doping is adjusted taking into account the life of a virtual element.
A method of evaluating a nitrogen-doped wafer characterized in that a decision is made as to whether or not this nitrogen-doped wafer can be employed as a wafer for semiconductor device manufacture by calculating the life of a virtual element on the nitrogen-doped heat-treated wafer.
A method of evaluating a wafer as set out above characterized in that the method of calculating the life of a virtual element on the aforementioned wafer is a TDDB test.
By xe2x80x9cvirtual elementxe2x80x9d is meant a simulated element structure manufactured when performing a TDDB test or TZDB test; by xe2x80x9clife of a virtual elementxe2x80x9d is meant the life of this simulated element structure.